Amorphous silica is mainly produced synthetically, particularly through a reaction between a sodium silicate solution and a mineral acid. Both react to produce silica in a precipitated form which is then dried and milled to the desired size. Sodium silicate (Na2SiO3) is normally produced at elevated temperature (around 1200 to 1400° C.) by reacting crystalline silica (SiO2) with sodium carbonate (Na2CO3). This process is expensive because of the large quantity of energy required and the cost of the reagents.
As an alternative raw material, magnesium silicate ore can be used to produce amorphous silica. For example, serpentine contains approximately 40 percent SiO2. Serpentine is a family of mineral silicates. The three most important serpentine polymorphs are lizardite, antigorite and chrysotile which is a form of asbestos. These all have essentially the same chemistry, but they differ in their structures.
Large quantities of serpentine are available in North America from the asbestos industry. Over the years, mountains of tailings, mainly lizardite Mg3Si2O5(OH)4, have accumulated. These tailings can also contain other minor components such as Mg(OH)2, Ni8Fe3, Fe3O4, etc. Such deposits represent an excellent, natural resource, easily available.
Amorphous silica is mainly used as an additive in a number of industrial applications, namely in the production of concretes, tires, paints, plastics, etc. Many of these applications are sensitive to silica's specific surface area, hydrophilic or hydrophobic properties, purity and particle size. Amorphous materials are characterized by atoms settled in irregular patterns (relative position and distance).
Activated silica can be produced by leaching magnesium silicate ore in hydrochloric acid media as described in GB 2 078 703. A material with a specific surface area of approximately 220 m2/g was achieved. During this hydrometallurgical process, the soluble portion passes in solution leaving behind the SiO2 phase along with other insoluble impurities. After this extraction step, silica is separated from the solution and washed. An additional cleaning step is necessary and it involves the separation of unreacted material by a physical method such as a shaker table. However, the level of impurities remains relatively high. This material is then dried, before or after a grinding operation, under conditions favoring the removal of moisture, whereas the chemically bound water is retained. The inventor observed a reduction of the material's specific surface area when the operations of separation, washing, and processing are not carried out immediately after the extraction, followed by a rapid drying step. No cause is identified yet to explain this and to determine which steps contribute to modify the surface.
To generate the properties of amorphous silica for its various industrial applications, production methods have to be flexible under controlled conditions. In addition, there are environmental benefits and economic advantages to produce this specialty silica from natural raw material. It is desirable, therefore, to elaborate an optimum method for the production of amorphous silica.